A Preliminary Investigation into the Practical Predictability of Convection Initiation during the Mesoscale Predictability Experiment (MPEX)

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Tuesday, 4 November 2014
Capitol Ballroom AB (Madison Concourse Hotel)
Alexandra M. Keclik, University of Wisconsin, Milwaukee, WI; and B. M. Burlingame, C. Evans, P. J. Roebber, G. Romine, and R. D. Torn

Convection initiation (CI) refers to the physical process of the formation of deep, moist convection. To first order, CI is triggered by a convergence mechanism, whether surface-based or elevated in nature, in a moist environment that is positively buoyant to upward parcel displacements. CI is a classic scale interaction problem requiring the favorable superposition of the synoptic-, meso-, and micro- scales. The synoptic-scale and meso-α scale establish the large-scale thermodynamic and kinematic environment favorable for CI; the meso-β scale contributes to horizontal variability within this environment; and the meso-γ and microscales contribute to crucial local boundary layer lifting and moistening. Consequently, CI is highly sensitive to the atmospheric state in which it occurs, particularly within the boundary layer. As a result, accurately predicting the timing and location of CI events poses a formidable challenge for storm-scale numerical simulations.

Motivated by studies demonstrating the influence of the synoptic- and meso-α scales upon deep, moist convection, the Mesoscale Predictability Experiment (MPEX) hypothesizes that the collection of non-routine synoptic- and meso-α-scale observations in the upstream, pre-convective environment across the Intermountain West and their subsequent assimilation into convection-permitting numerical forecasts significantly improves forecasts of the timing, location, and mode of convection initiation and evolution downstream. Herein, utilizing output from two thirty-member ensembles of Ensemble Kalman filter-initialized, convection-permitting real-data numerical simulations, one incorporating MPEX observations and one not, we test the related hypothesis that a more intensively-sampled representation of the pre-convective atmospheric state is sufficient to improve the practical predictability of initial (or pristine) CI timing and location over the range of fifteen events sampled by MPEX between 15 May and 15 June 2013.

Observed and simulated radar reflectivity data exceeding 35 dBz at the -10 C level is used to identify observed and numerically-simulated initial CI events for each MPEX event. Subsequently, probabilistic, temporally-binned spatial verification methods are utilized to examine forecast skill. For a given event, the timing and location of the initial observed and simulated (by each ensemble member) occurrence of CI are first identified. The probability of CI occurring within S km of each model grid point within a T h window is defined as the number of ensemble members within each ensemble correctly forecasting CI within these thresholds divided by the total number of ensemble members. Spatiotemporal thresholds considered are 40 km/1 h, 80 km/1.5 h, 120 km/2 h, 160 km/2.5 h, and 200 km/3 h. The forecast skill of each ensemble is evaluated at these spatiotemporal thresholds using a Brier Skill Score (BSS) computed with reference to the sample climatology. Results from one or more MPEX events will be presented. Connections to previous deterministic and probabilistic investigations into CI predictability will be drawn as appropriate.